Converting your basement into a legal rental unit in Washington, D.C. is a great way to create an income stream to help pay off a mortgage. In our last blog post we talked about the requirements for converting your basement into a rental unit and one of the key points was headspace.
If your existing basement is below the 7 foot mark, the only way to gain more headspace is to dig down. There are several options for lowering the concrete slab and the first step in determining the right path is to do some research to see what the conditions are of your existing footers.
To find the bottom of the existing footer, a minimum of two test pits will need to be dug, typically at opposite corners of the house. A soil engineer will then test the bearing capacity of the soil, which needs to be submitted as part of the permitting process. Depending on the depth of the footer and the desired finished ceiling height (no less than 7 feet), we work with a structural engineer to determine the best option for lowering the slab. Within a week of these tests, you will know what will be required.
There are three main options for digging down once the research is complete:
SLAB REPLACEMENT
If the existing footers are deep enough, the existing slab can be removed, the dirt excavated, and a new slab poured at a greater depth. This does not require any underpinning and would be the least expensive option. This option is typically considered when the original slab is uneven or out of level and it impedes on the 7 foot clearance.
MODIFIED UNDERPINNING OR 'BENCHING
If cost is a major factor and the existing footers don’t meet the requirements, modified underpinning or benching may be the next best option. In this scenario, new benched footers, integrated into the existing footers, would be poured in order to lower the slab. The downside to this method is that it creates what would look like an actual bench around the perimeter of the basement. This would reduce the overall footprint at floor level, but walls can be built on top of the footer and creative design can minimize the impact of these benches and render them useful, whether for built-in storage, or actual seating in an entryway, for example.
UNDERPINNING
If modified underpinning isn’t an option for you, traditional underpinning is the way to go. This method is the most expensive and time consuming of the three. With traditional underpinning, the dirt is actually dug out below the existing footers and new footers are poured. To minimize the chance of collapse, a structural engineer develops a digging plan and creates a map of the basement, dividing it into two-foot sections. As an example, every 4th section is dug and poured simultaneously. Once the concrete has cured, the next sections are dug and poured until the whole foundation is reinforced.
After the slab-lowering method has been determined, there are several other requirements to obtain permits. Plumbing is a major one as the drain lines run under the slab and connect to the city’s sewer line in the street. Stairs are also an important consideration: when the slab is lowered, the rise changes between floors affecting the head clearance.
With the hard part finished, the floor plan and overall design of the unit can be drawn up and submitted for permitting. This can be a long and labor intensive process, but at the end of the day, you’ll have a whole new level of livable space in your home, an invaluable asset.
